1. Field of the Invention
The present invention relates to a high strength titanium material having an improved ductility, and a method for producing same. More particularly, it relates to a high strength titanium material having an improved ductility which is obtained by defining the contents of nitrogen (N), iron (Fe), and oxygen (O) under a constant condition, and a method for producing same.
2. Description of the Related Art
Various alloys containing Al, V, Zr, Sn, Mo, etc., are well known as a high strength titanium alloy. Of these high strength titanium alloys, a Ti--6Al--4V alloy; a high strength titanium alloy having an improved toughness, for example, a Ti--5Al--2Sn--2Zr--4Cr--4Mo alloy; and a high strength titanium alloy having an improved ductility, for example, a Ti--15V--3Cr--3Al--3Sn alloy, are well known. But, since such high strength and high toughness or ductility titanium alloys can be obtained only by a combination of special and strict controls of an alloy composition, and hot working or after a heat treatment, etc., the production method is complicated and costly.
If a high strength titanium material having substantially the same properties as that of the high strength titanium alloy can be obtained, without the necessity for a large amount of alloy composition and complicated treatments, such an alloy can be widely used in many fields.
Japanese Unexamined Patent Publication (Kokai) No. 61-159563 discloses a method for producing a forged material having a tensile strength of 80 kgf/mm.sup.2 or more using an industrially pure titanium, by which the above-mentioned object is satisfied, and when crystal grains are refined by the above method, a high strength, pure titanium forged article having an improved ductility can be obtained. Nevertheless, this process requires a hot forming in which only a forging forming method, such as an upsetting or a heavy working, is used.
Therefore, a high strength titanium material which can be worked to form various shapes by using a usual production method, e.g., a plate rolling such as a hot strip rolling, a bar rolling, or a wire rolling without using the above-mentioned special forming method has been developed. Accordingly, the present invention is related to a various-shaped article of a titanium material produced by the above-mentioned production methods. Particularly, the high strength titanium material produced by a bar rolling process will be explained hereinbelow.
Table 1 shows examples of the relevant Japanese Industrial Standard (JIS) and an ASTM Standard.
As shown in Table 1, the standard material for the highest strength industrially pure titanium is that of ASTM G-4, having a tensile strength of 56 kgf/mm.sup.2 or more.
The N, Fe, and O, etc., shown in Table 1 are impurities, the upper limit of the content of which is defined. In producing a titanium material, the relationship between the contents of such elements and the mechanical property values, the relationship between metallurgical behavior of such elements and the microstructure, and further, the effects on the above-mentioned items of a heat treatment working condition during production must be clearly defined.
TABLE 1 __________________________________________________________________________ Mechanical properties Chemical Composition (% by weight) Tensile Total Strength Remaining (min) Ductility N C H Fe O Impurities kgf/mm.sup.2 (min) % (max) (max) (max) (max) (max) (max) Ti __________________________________________________________________________ JIS 1 28 27 0.05 -- 0.015 0.20 0.15 -- Rest ASTMG-1 24.5 24 0.03 0.10 0.015 0.20 0.18 0.03 " JIS 2 35 23 0.05 -- 0.015 0.25 0.20 -- " ASTMG-2 35 20 0.03 0.10 0.015 0.30 0.25 0.03 " JIS 3 49 18 0.07 -- 0.015 0.30 0.30 -- " ASTMG-3 45.5 18 0.05 0.10 0.015 0.30 0.35 0.40 " ASTMG-4 56 15 0.05 0.10 0.015 0.50 0.40 0.40 " __________________________________________________________________________